1. Field of the Invention
The invention is related generally to the field of interpretation of measurements made by well logging resistivity instruments for the purpose of determining the properties of earth formations. More specifically, the invention is related to an apparatus and method for determination of the electrical resistivity of borehole mud.
2. Background of the Art
Modern conventional electrical resistivity measurements are grouped into two classes, those injecting electrical currents into the formation by means of electrodes (galvanic logging devices, including lateral, spherically focused, and normal devices) and those using coils (induction logging devices) for creating eddy currents in the formation. The galvanic logging methods are really just developments of the original electrode instrument methods invented by the Schlumberger brothers in the 1920""s. (L. Allaud and M. Martin, xe2x80x9cSchlumberger, the History of a Technique,xe2x80x9d Wiley, New York, 1977). The induction logging methods and devices were created by Henri-Georges Doll in the 1940""s. (H. G. Doll, Pet. Trans. AIME, 186, 148, 1947).
For the induction logging device the signal measured from a particular formation zone is inversely related to the resistivities in the formation around the borehole and to the resistivities within the borehole. For the galvanic logging devices the signal measured is non-linearly related to the resistivities and the resistivity contrasts of the borehole and the formations surrounding the borehole. At the borehole/formation/invaded zone boundaries, electrical charges arise for galvanic logging devices, while for induction logging devices, the effect is due to the induced current in the borehole/formation/invaded zone media. In the field of electromagnetic surveying, the different situations are usually described as different modes, the transverse electric (TE) mode in the case of the induction devices, and the galvanic or transverse magnetic (TM) mode for the galvanic (electrode) devices.
FIG. 1 (Prior art) is a standard diagram showing the environmental and formation conditions usually considered for determining whether to select an induction logging instrument or a lateral logging instrument when logging a specific well. For instance, the resistivity of the formation water Rw is estimated using, for example, information from a nearby well. The resistivity of the mud filtrate Rmf is then estimated, which can also be done from information from a nearby well if the mud system will be the same, or by measurement of a sample if one is available. Then the porosity of the formation is estimated, which can be based on prior known information of the porosity obtained from a nearby well or from other logs that measure porosity. Using the estimated data for Rw and Rmf, a calculation of the ratio Rmf/Rw is made. By referring to a chart or diagram similar to the one shown in FIG. 1, it is possible to determine the type of instrument most suited for the particular well.
An example model formation used in interpretation of resistivity measurements is shown in FIG. 2 (prior art). A borehole 90 of diameter Dbh is shown penetrating a formation of interest 92. The xe2x80x9cinvaded zonexe2x80x9d of the formation invaded by the drilling mud fluid is shown at 94, which, in this example, has a step profile of diameter Di. The resistivity of the drilling mud itself in the borehole 90 is shown as Rm, the resistivity of the invaded zone as Rx0 and the resistivity of the formation as Rt. The measurements made by the logging instrument are then used to derive the formation resistivity.
The measurements have to be corrected for the effects of a mud-filled wellbore. In order to do the corrections, it is often required to know the value of a resistivity of the mud in the wellbore, and the mud resistivity value must be known with good accuracy. The wellbore penetrates a formation, and the formation has its own resistivity value. Very often, there is a large difference between the resistivity of the mud in the wellbore and the formation resistivity. As the formation to mud resistivity contrast increases for a particular depth in the wellbore, certain undesired perturbations can be seen in the output signals produced from the well logging apparatus in the wellbore. As a result, correcting these perturbations, which exist in the output signals from the induction well logging apparatus, becomes mandatory, especially when the well logging apparatus is logging large wellbores.
There are three types of devices used for determining the mud resistivity Rm. In the first type of system, the apparatus is specifically designed to measure the mud resistivity. U.S. Pat. No. 5,574,371 to Tabanou et al discloses a measurement probe that includes a bottom electrode disposed on a bottom of the probe when the logging apparatus is disposed in the wellbore, a second electrode, and at least one measurement electrode disposed adjacent the bottom electrode for measuring a voltage potential drop in a region of the mud which is disposed directly below the bottom electrode of the measurement probe when the probe is disposed in the wellbore. When the measurement probe is energized, a current flows in the mud between the bottom electrode and the second electrode. When the current is initially emitted into the mud from the bottom electrode, and when the current is received from the mud in the bottom electrode, the current flows in a direction which is approximately parallel to a longitudinal axis of the logging apparatus tool string. Since the measurement electrode is disposed adjacent the bottom electrode, the measurement electrode measures the voltage potential drop in a region of the mud disposed directly below the bottom electrode. In addition, the voltage potential drop in such region measured by the measurement electrode is controlled primarily by the current being emitted from or received in the bottom electrode and flowing in such region.
The second type of apparatus used for measuring mud resistivity uses a modification of prior art galvanic sondes. Such a device is disclosed by Eisenmann (U.S. Pat. No. 6,046,593). A typical configuration uses a current electrode disposed between a pair of measure electrodes. Additional guard electrodes inject current into the formation and maintain focusing of the current from the current electrode. The mud resistivity is derived by measuring the potential difference between the measure electrodes and by measuring the current in the current electrode.
The third method for determining mud resistivity is based upon inversion of data from resistivity sondes. When such methods are used, the mud resistivity becomes one more parameter in the inversion process. This increases the number of variables for the inversion and may also reduce the level of confidence in the results. This makes it desirable to have independent methods of obtaining mud resistivity.
The results obtained by Tabanou et al show an error of 7-8% in estimation of mud resistivity. Accurate measure of mud resistivity is becoming more critical to properly correct resistivity values obtained with induction and galvanic resistivity instruments. This is particularly important with transverse induction logging tools where the signal to noise ratio of the cross components may be lower than for the principal components. The present invention satisfies this need with an order of magnitude increase in accuracy of the Rm measurement values.
The present invention is an apparatus conveyed in a borehole for determination of a resistivity of a borehole fluid. A pair of spaced apart current electrodes and a pair of spaced apart measure electrodes are disposed in a recessed portion of the tool between a pair of guard electrodes. Insulators are provided between the measure electrodes, between the measure electrodes and the current electrodes, and between the current electrodes and the guard electrodes. Due to the recess and due to the guard electrodes being at the same potential, the current between the current electrodes in the borehole fluid flows substantially parallel to the borehole axis and is relatively insensitive to the formation resistivity. A processor is used to determine an apparent mud resistance from the measure current and the voltage difference between the measure electrodes. The apparent mud resistance is then used to determine the mud resistivity. In an alternate embodiment of the invention, the same pair of electrodes is used as current electrodes and measure electrodes.